Tech

Smiths digital instruments are made in the UK in the last surviving original factory unit. The Smiths brand offers a range of digital speedometers and tachometers. The classic replica units were brought to the classic market in 2012 and have been designed to work with both negative and positive earth, 12v systems.

The Smiths digital classic units are available in the original grey and black face versions that are similar to those fitted to classic British motorcycles of the era and are available in both MPH and KMH versions, both in the standard fit and in a slightly larger case to fit the new Hinckley Triumph bikes.

The Tachometers are either wired into the switched side of the ignition coil circuit or may be driven by a dedicated ECU signal if one is available and has programmable options from single to six cylinders.

Read the Motorcycle Classics “How To” Article:
Install a Smiths Digital Speedometer
April 13th 2015 – By the Motorcycle Classics staff

The Speedometers operate digitally, and use a signal from either a dedicated ecu output (modern bikes) or from a sensor such as a “hall-effect” sensor hidden inside a dummy cable (to maintain the profile and look of your classic bike) or a moving magnet type (reed switch) similar to that used on a bicycle computer. See the accessories page for details on senders for your bike.

The Speedometers have a dedicated power circuit to supply a 5v power feed to the sender but can also be used in conjunction with 12v hall-effect senders or reed switches.

The Smiths Classic Speedometer has a very handy “Drive to set” function that makes the unit extremely easy to set up, simply driving the bike over a measured mile or kilometer, will enable the unit to calibrate itself and record and display speed and distance correctly.

The units will work on positive, as well as negative earth systems.

…will they work on my bike?
The units will work on any machine with a 12v supply and a conventional ignition coil for the tacho pick-up.
The wiring of the tachometers is also compatible with dedicated ECU rpm signals.
It is also possible to use a HT signal amplifier to drive the tach for ignition systems with “exciter” type coils or magneto driven igntion systems, the tach units will still require a 12v supply.

….how do I set the units for rpm?
The tacho comes pre-set for twin cylinder engines but with simple setup procedure to enable the unit to be set for singles, triples, fours, five and six cylinder machines, follow the instructions to obtain the correct read out for your bike.

….what is a hall-effect sensor?
A simple electronic sensor that generates a pulse so the receiving unit can interpret the revolutions of the wheel or drive unit, just like on a bicycle speedo/computer. We have a wide range available styled to look like the original speedo cable that fit directly into a wheel-hub mounted speedo drive or gearbox mounted output.

….do I need a sensor for my Smiths speedo?
Yes, if you not have a dedicated ECU output signal (modern machines) then you will need one of the hall-effect or reed switch senders available on the accessories page

….how long are the looms on the speedo, tach and sender units?
The Smiths digital range of speedos and tachs come with approx .5m of loom, the senders and sensors have approx 1m of cable/loom.
Most motorcycles use a front wheel or gearbox sprocket output for speedometer activation, the senders/sensors are designed accordingly.
As the sender/sensor is not a “drive cable” it does not need to follow the original route of the speedo cable, this provides owners with the widest choice of installation locations.
Our test bikes, along with many owners preference use a junction box under the riders seat allowing for easy access and weather proofing, there is no requirement to wire to the headlamp case.
Each installation is different and most will only need slight extentions to the supplied loom if any but as the units are also fitted to trikes and classic cars we understand some owners may require longer loom adaptions.
Any extensions to the loom either for the sender or the Smiths digital speedos or tachs should use good quality automotive type cable size .5mm² or (better) multi-core or trailer cable is ideal for neatness.

….my bike is a custom, I don’t have a speedo drive or ecu signal?
We would advise you use a reed switch sender as available on the accessories page These senders can be configured almost anywhere on the bike (or project) close to a revolving wheel component or final drive sprocket. This type of sender uses a revolving magnet to trigger and careful siting of the sender unit and revolving trigger magnet is essential for reliable performance. Our senders are supplied with one trigger magnet which will be fine for wheels up to 19in (48cm) rim size, more magnets may be used to give a smoother speedo reading at speeds below 15mph (25kmh). wheel sizes over 19in (48cm) should use a minimum of two trigger magnets to ensure smooth speedo performance at low speeds.
We always recommend fitting to the rear wheel/swing arm for reliabilty and stability of signal

….what size are the units, will they fit my bike?
The units are all digital versions of the original mechanical designs and all external dimensions are replicated in the new digital units

Smiths digital chronometric and classic dimensions

…..and how do I set the speedo for mileage/km?
Instructions are included to setup your Smiths digital speedo using the wheel circumference, or more conveniently, the smiths digital range available from Puca has a “drive-to-set” mode, follow the simple instructions and simply ride the bike over a measured mile or km and the unit calibrates itself. Please see the “Drive to set” section on the top right of this page.

….are there any functions on the speedo or tach?
For practicality and weatherproofing the Smiths digital range of motorcycle units do not have case mounted switches.
The Smiths digital speedos and tachs use a single wire within their looms to allow setup and changes of operation mode by means of a momentary contact to the bikes negative (-) circuit.
The Smiths digital tachometer will only require initial setup using the toggle function.
The speedometer uses the toggle function during operation to provide access to the trip function and in the case of the Smiths digital chronometric, to toggle the “ticking” display mode.
For riders safety sake, these toggle operations should normally only be operated whilst the machine is stationary, a handle bar mounted momentary type switch (such as a horn or ignition cut-out type button) may be used, as may an under-seat located momentary switch or simply by means of a momentary touch of the toggle wire to negative (-)

– Content and Imagery Reprinted with Permission of Smithsspeedos.co.uk

Converting a mechanical speedo drive to an electronic drive

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